As is known, in some activities and sports practices the perceptive capacity and, more generally, the vision of an object may be disturbed in the presence of a high relative velocity between the perceiving subject and the object, or in the presence of poorly defined objects and surfaces due to a limited illumino-environmental contrast.
Conversely, in some circumstances, vision may be further disturbed and impaired when it is necessary to focus alternately and repeatedly an object in the presence of an illumino-environmental contrast now limited now high, such as for instance that which exists, respectively, between an object and the ground or an object and the sky that has in itself a markedly higher luminance.
In this regard, some experimental tests carried out by the Applicant in sunny conditions in a varied town environment in the presence of vegetation and at the latitude of about 43.degree. North, have shown that the luminance of the sky may be from 1.5 to about 9 times higher than the luminance of the ground.
In such circumstances, the eye must vary continuously the pupillary diameter, consequently adjusting the exposure of the retina to the external light, to face alternately high and low luminosity conditions.
In spite of the efforts of the eyes to adapt itself to an average situation, which in any case would not allow an optimal vision, this brings about a visual stress that may lead to a visual straining and in some cases also to a partial (even though reversible) damage of the receptors located on the retina.
Such unfavorable phenomenon is always present and particularly marked when playing some sports activities, among which golf or clay-pigeon shooting, in which the athlete or the sportsman repeatedly passes from the observation of objects having a low illumino-environmental contrast or at least that are in a relatively little luminous field (for instance, the golf ball against the background of the green course) to the observation of objects that are in a markedly more luminous field (for instance, the flight of the golf ball against the blue background of the sky after each shot).
Examples of other activities subject to high visual stresses due to the need of perceiving rapidly moving objects include winter sports in general, as well as tourist- or competitive driving of any vehicles, such as for instance, cars or motor-cycles, aircrafts, boats, etc.
In order to somehow obviate the problem and to render the vision of moving objects clearer and sharper, it has been proposed the adoption of protective optical elements made of plastics--such as for instance eyeglasses or unitary masks--incorporating a specific substance suitable to filter visible light, as is described in European patent application EP 0 382 684.
Even though the protective optical element disclosed in the aforementioned European patent application can partly compensate for the anomalies due to the limited illumino-environmental contrast, allowing to reduce the perception time of moving objects and to improve the definition of their contour, it does not allow to remedy in any way to the visual stress due to the need of repeatedly take account of different luminance conditions.